1. Field of the Invention
This invention relates to an improved, sag resistant tungsten filament and its use in lamps. More particularly, this invention relates to a tungsten filament being at least about 85% recrystallized and having a microstructure comprising a large, elongated and interlocking grain structure, a method for producing same and its use in electric lamps.
2. Background of the Disclosure
The use of tungsten filaments in electric lamps, such as incandescent lamps, is well known and old to those skilled in the art. The efficiency or efficacy as well as the light output and color rendering ability of an incandescent lamp is very much dependent on the temperature at which the filament operates. The filament temperature also determines the quality of the emitted light. Generally, the more efficient incandescent lamps, such as tungsten-halogen lamps, employ filaments in the form of coils or helixes and more particularly coiled-coils or double helixes in which the filaments are operated at temperatures of about 2500.degree. C. In stage and studio lamps the filaments are operated at temperatures as high as 2900.degree. C. Higher filament temperatures permit the use of smaller size filaments and concomitantly smaller size lamps for a given light output, which is very desirable in the market place. At the present time, the use of filaments at temperatures above about 2300.degree. C. results in substantial sag which, in turn, distorts the filament coil resulting in an increase in the radiant heat loss, thereby decreasing the luminous efficacy. Sag can also result in shorting across various portions of the coil. Tungsten ingots intended for making tungsten filaments contain a very minor amount of dopants such as potassium, aluminum and silicon. In general, tungsten ingots used to produce wire from which filaments are made consist essentially of from about 99.95 to about 99.99 wt. % of tungsten, along with minor amounts of one or more dopants and impurities.
In fabricating the fine tungsten wire from which filaments are produced, a number of rolling, swaging, wire drawing and annealing steps must be employed. In fabricating filaments from wire, either single coil filaments or coiled-coil filaments in which there is a secondary coil, it is common practice to heat the resulting filament structure at a temperature generally ranging from about 1300.degree.-1600.degree. C. for a period of from about 1 to 10 minutes in order to slightly anneal and stress relieve the so-formed filament. This results in a filament having an essentially unrecrystallized, fibrous microstructure such as is disclosed, for instance, by Smithells on pages 136-137 in his book "Tungsten", published in 1952 by Chapman and Hall, Ltd. (London). Such a fibrous microstructure results in a relatively weak filament having extremely little, if any, sag resistance at the 2000.degree. C. plus temperatures at which filaments are operated. Accordingly, those skilled in the art know that such filaments have to be recrystallized such as is disclosed, for example, by Smithells on pages 136-145 and in U.S. Pat. Nos. 3,927,989 and 4,296,352. Both of these patents disclose that tungsten wire filaments normally recrystallize at a temperature in the general range of between about 1900.degree.-2500.degree. C. The most ideal filament would be one formed of a single crystal of tungsten or one that was recrystallized in a manner so as to form a single crystal of tungsten. Such a filament would have the maximum possible sag resistance and tensile strength. However, at the present time no one has been able to make such a filament and there is still a great need in the art for filaments of improved high temperature sag resistance for use in more compact and efficient lamps.